Biphenyl (also
called Diphenyl, but exactly one of two compounds. The other is o-phenylphenol,
used to prevent the growth of moulds ) is an aromatic hydrocarbon; molecule
structure is composed of two six-sided carbon rings connected at one carbon site
on each ring. Pure biphenyl is a toxic colourless crystalline solid with a
pleasant odour; melting point 70 C, boiling point 255 C, which gives plates or
monoclinic prismatic crystals; it is insoluble in water but soluble in ordinary
organic solvents. It is directly used in the preservation of citrus fruits as a
fungistat in transportation containers. It is a raw material for polychlorinated
diphenyls (PCB) in which chlorine replaces hydrogen in biphenyls. There are 209
chlorinated isomers of biphenyl theoretically. But PCBs are referred to the
biphenyl compounds with one to ten chlorine substitutions. PCBs are used
heat-transfer agents and as electric insulators that block the flow of electric
current across in electrical equipments. They are known as environmental
pollution materials which are accumulated in animal tissues and cause toxic
effects including carcinogenesis. Biphenyl is used as an intermediate for the
production of a wide range of organic compounds (e.g. emulsifiers, optical
brighteners, crop protection products, plastics), as a heat transfer medium
alone or with diphenyl ether in heating fluids, as a dyestuff carrier for
textiles and copying paper and as a solvent in pharmaceutical production.